A simple multi-module Maven project with which exec-maven-plugin is unusable.

.gitignore

.idea/
*.iml
target/

HelloPrinter.java

public final class HelloPrinter {
    void printHello() {
        System.out.println("Hello world!");
    }
}

MyMain.java

public final class MyMain {
    public static void main(String[] args) {
        new HelloPrinter().printHello();
    }
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <artifactId>exec-multi-module-test</artifactId>
        <groupId>com.mkscrg.sandbox</groupId>
        <version>1.0</version>
    </parent>

    <artifactId>module1</artifactId>
</project>

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <artifactId>exec-multi-module-test</artifactId>
        <groupId>com.mkscrg.sandbox</groupId>
        <version>1.0</version>
    </parent>

    <artifactId>module2</artifactId>

    <dependencies>
        <dependency>
            <groupId>com.mkscrg.sandbox</groupId>
            <artifactId>module1</artifactId>
            <version>${project.version}</version>
        </dependency>
    </dependencies>
</project>

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.mkscrg.sandbox</groupId>
    <artifactId>exec-multi-module-test</artifactId>
    <version>1.0</version>
    <packaging>pom</packaging>

    <modules>
        <module>module1</module>
        <module>module2</module>
    </modules>

    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.2.1</version>
            </plugin>
        </plugins>
    </build>
</project>

run.sh

#!/usr/bin/env bash

# StackOverflow question: http://stackoverflow.com/questions/11091311/maven-execjava-goal-on-a-multi-module-project

mvn exec:java -Dexec.mainClass=myMain